1. Field of the Invention
This invention relates to a ferromagnetic iron oxide powder for magnetic recording, specifically, to a ferromagnetic iron oxide powder having high coercive force and improved stability to pressure and heat.
2. Description of the Prior Art
Ferromagnetic iron oxides for magnetic recording are required to have high coercive force and exhibit superior orientability.
One effective means of increasing the coercive force of a ferromagnetic iron oxide powder involves incorporating cobalt ions in iron oxide to form a solid solution, and various methods of this type are suggested, for example, in U.S. Pat. Nos. 3,117,933 and 3,671,435, Japanese Patent Application (Laid Open) No. 101599/73, and Japanese Patent Publication Nos. 6538/66, 4264/74, 27719/66 (corresponding to U.S. Pat. No. 3,573,980), 15759/73, 10994/73 and 6113/67. Magnetic recording media, such as magnetic tapes, made by using the cobalt-containing iron oxides produced by such methods, however, have the defect that they are unstable to pressure and heat, and the magnetic signals recorded become weak or are transferred to a great extent.
Another method for increasing coercive force comprises adhering a layer of a cobalt compound or cobalt ferrite to the surface of magnetic iron oxide powder free from cobalt in the form of a solid solution, or growing such layer on the surface thereof, and such is specifically disclosed, for example, in Japanese Patent Application (Laid Open) Nos. 108599/74, 37667/75 and 37668/75 and Japanese Patent Publication No. 49475/74. Ferromagnetic iron oxide powders obtained by these methods are stable to pressure and heat and have improved transfer characteristics as compared with those obtained by dissolving cobalt.
On the other hand, Japanese Patent Application (Laid Open) Nos. 22707/72, 1998/73, 51297/73, 54497/73, 58398/73, 76097/73, and 87397/73, Japanese Patent Publication No. 20557/75 disclose a method in which cobalt or at least one other metal is adhered onto ferromagnetic iron oxide dispersed in water, in the presence of a reducing agent. This method is designed to reduce metal ions such as cobalt to cause them to adhere to the ferromagnetic iron oxide. Such a method is different from the present invention wherein ferromagnetic iron is reduced, and then, in the absence of the reducing agent, a metal ion such as cobalt is added.
Japanese Patent Publication No. 17,113/64 discloses that a Co-doped iron oxide cooled in a magnetic field can be treated with a solution of sodium borohydride. Since this treatment is carried out after cobalt doping, and is intended to fix the magnetic field cooling effect, it is clear that the present invention differs from this prior invention.
The method comprising adhering a cobalt compound to the surface of a ferromagnetic iron oxide not containing cobalt can provide a product having a coercive force of about 500 to 600 Oe at the highest. On the other hand, according to the method involving growing a cobalt ferrite layer, the coercive force of the resulting product is nearly proportional to the cobalt ferrite layer grown on the surface. The cobalt ferrite layer is a magnetic oxide in which cobalt is dissolved to form a solid solution, and is unstable to pressure and heat. For this reason, this defect of the cobalt ferrite layer grown on the surface is gradually exhibited, and this method cannot provide a ferromagnetic iron oxide powder having high coercive force and improved stability to pressure and heat.
Iron oxide having a high coercive force obtained only by a precipitation reaction in aqueous solution has better thermal stability and more stable properties with the passage of time, for example, SP ratio, than cobalt-containing iron oxide produced from cobalt-containing goethite, or ferromagnetic iron oxide contained by adhering cobalt to iron oxide and heat-treating it at a temperature as high as 300.degree. to 500.degree. C to thereby diffuse the cobalt and increase the coercive force. Furthermore, since it can be produced by a simple process step, it is suitable for commercial production.
The coercive force, however, does not increase proportionately to the increased amount of cobalt added, and it is extremely difficult to raise the coercive force beyond 500 to 600 Oe. Thus, the coercive force of such a ferromagnetic iron oxide is not entirely sufficient for use in high density recording magnetic tapes. The present invention relates to an improvement in the above-described method of reaction in an aqueous solution, and insures the production of ferromagnetic iron oxide having good magnetic stability and high coercive force.